GENETIC-DIVERGENCE, REPRODUCTIVE ISOLATION, AND MARINE SPECIATION

In marine species, high dispersal is often associated with only mild genetic differentiation over large spatial scales. Despite this generalization, there are numerous reasons for the accumulation of genetic differences between,large, semi-isolated marine populations. A suite of well-known evolutionary mechanisms can operate within and between populations to result in genetic divergence, and these mechanisms may well be augmented by newly discovered genetic processes. This variety of mechanisms for genetic divergence is paralleled by great diversity in the types of reproductive isolation shown by recently diverged marine species. Differences in spawning time, mate recognition, environmental tolerance, and gamete compatibility have all been implicated in marine speciation events. There is substantial evidence for rapid evolution of reproductive isolation in strictly allopatric populations (e.g. across the Isthmus of Panama). Evidence for the action of selection in increasing reproductive isolation in sympatric populations is fragmentary. Although a great deal of information is available on population genetics, reproductive isolation, and cryptic or sibling species in marine environments, the influence of particular genetic changes on reproductive isolation is poorly understood for marine (or terrestrial) taxa. For a few systems, like the co-evolution of gamete recognition proteins, changes in a small number of genes may give rise to reproductive isolation. Such studies show how a focus on the physiology, ecology, or sensory biology of reproductive isolation can help uncover the genetic changes associated with speciation and can also help provide a link between the genetics of population divergence and the speciation process.